1. Field of the Invention
The present invention relates to communications, more particularly to modem communications.
2. Description of the Prior Art
Modems communicate over channels such as telephone lines. These channels suffer from channel impairments such as echoes that are created by impedance mismatches. When modems operate in full duplex mode, echoes of the modem's transmitted signal interfere with the signal to be received.
One approach to alleviating this problem is to use frequency division multiplexing. Frequency division multiplexing decreases the channel impairment related to echoes, by transmitting signals in one portion of the communication channel's bandwidth and receiving signals in another portion of the communication channel's bandwidth. The echo is removed from the received signal by simply filtering in the frequency domain. U.S. Pat. No. 4,890,316 entitled "Modem for Communicating At High Speed Over Voice-Grade Telephone Circuits" discloses such a technique. Unfortunately, this technique reduces transmission speed because the modems cannot use the entire bandwidth of the communication channel for both transmitting and receiving signals.
Another approach to reducing the effect of echoes is to use a modem that cancels echoes. U.S. Pat. No. 5,007,047 entitled "Adaptive Rate Control for Echo Cancelling Modem" discloses such a technique. This technique offers the advantage of using the entire communication bandwidth for both transmitting and receiving; however, it is limited by a modem's ability to cancel echoes. After cancelling an echo, the remaining residual echo interferes with the modem's ability to receive signals and thereby limits high speed data reception.